1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a sheet, and a fixing device that fixes a toner image on the sheet.
2. Description of the Related Art
In the image forming apparatus, a toner image is fixed on a sheet surface by the fixing device which heats and applies pressure to the toner image formed on the sheet. When the sheet carrying the toner image passes through the fixing device, the sheet is arched on an image formed side due to heat contraction of the toner (hereinafter this phenomenon is referred to as toner curl). On the other hand, at a marginal portion of the sheet where there is no toner image or at a portion of the sheet bearing a relatively small amount of toner, moisture is evaporated from the sheet due to heat of the fixing device which results in an arch formation in a direction opposite to the image formed surface (hereinafter this phenomenon is referred to as heat curl).
Dimension, shape, and direction of the two curl types change depending on various factors such as shapes of conveyance path inside the image forming apparatus, sheet size or a sheet making direction, fixing temperatures of the fixing apparatus, and toner type. Moreover, in addition to the above factors, the curl form changes depending on the toner image type and is also influenced by the environment where the apparatus is in use.
When the sheet is curled, a paper jam can occur at the conveyance path downstream of the fixing device or loading failure can occur due to the curl of the discharged sheet. Moreover, the image forming apparatus can have two-sided conveyance path for conveying a sheet to the image forming unit again to form the image on a second surface of the sheet after the image is formed on a first surface. In such apparatus, transfer failure may occur due to the curl, or the sheet fails to enter the fixing device, when an image is transferred onto the second surface.
Conventionally, a method of correcting the curl has been proposed in which a curl correcting unit is provided downstream of the fixing device for correcting the curl generated in the fixing device, and the transfer or conveyance failure downstream of the conveyance path is reduced (refer to Japanese Patent Application laid-open No. 9-301599, corresponding to U.S. Pat. No. 5,905,934). As a method of dealing with the differences in the curl level depending on the paper types and environments, there is an apparatus configured to adjust the curl correcting capability (refer to Japanese laid-open Application No. Hei 8-169615) or make the apparatus attachable/detachable. Moreover, there is a known mechanism to correct the two curl types (i.e., the toner curl and heat curl). Since both curls are generated in the opposite directions, in order to correct both curls, the downstream of the conveyance path in the fixing device is diverged, and the curl correcting units operating in opposite directions are disposed in the respective conveyance paths.
In the conventional apparatus, between the two curl types described above, the effect of the heat curl is especially large, and therefore, the curl correction is performed to all recording media mostly by disposing the curl correcting unit downstream of the fixing device. Nevertheless, since the curl formation varies depending on the sheet types and images as described above, there are cases where the curl cannot be corrected depending on the sheet types, or in some cases, the curl is corrected more than necessary. Especially, along with recent emergence of the high-speed apparatus market, the fixing temperature is set higher than before, and the applied pressure is being increased. As a result, a level of curl tends to get higher than the conventional machines, especially because the curl of the thin sheet becomes larger. This causes the problem that the sheet cannot enter the fixing device when the image is fixed on the second surface in a two-sided printing. Accordingly, the curl must be corrected and reduced.
Further, there is a problem that a configuration for skew registration employed for high speed operation shows a tendency to be weak against the curl compared to the conventional registration configuration. Moreover, in addition to the high speed, along with rising request for high image quality, in order to remove a trace on the image, a long roller is frequently disposed downstream of the fixing device. The long roller has a cylinder portion which is longer than the sheet width. However, in the case of employing the long roller, a gap is formed between a guiding board and the roller, so that paper jam occurs if the level of curl is not smaller than in the conventional machines.
In this regard, for example, when a curl correction unit in the two-sided conveyance path is devised to correct curling, large pressure must be applied to correct a thin paper at a lowered temperature. However, by applying large pressure, an image surface of the sheet may be damaged or wrinkled. Moreover, for example, when a curl correcting unit corrects a curl immediately after the fixing device, it is possible to apply a large curl to the sheet because the sheet temperature is high. However, depending on the sheet types, the curl can be generated more than necessary, or the image can be damaged if a large pressure is applied immediately after fixing. Adjustment of the curl correcting capability or the attachment/detachment mechanism is employed to deal with changes in the level of curls depending on the sheet types. Furthermore, the attachment/detachment and adjustment between the sheets are performed by a motor or solenoid in the high speed apparatus. However, it is not easily implemented because the apparatus and sequence become complex, and good durability is not obtained.
On the other hand, a number of sheet types used by the image forming apparatus is increasing. Therefore, it is difficult to fix the image with stability on all kinds of sheets in the image forming apparatus in which an image is fixed using a single fixing device. Therefore, a configuration is considered in which an image is fixed on the sheet using a plurality of fixing devices which are aligned serially in the conveyance path (refer to Japanese Patent Application laid-open Nos. 5-158364 and 7-191564). Also in this image forming apparatus that employs the plurality of fixing devices to secure stable fixing, there appears the problem relating to the curl correction as described above when the sheet conveyance is carried out at high speed.